Silver powder is used in the electronics industry for the manufacture of conductor thick film pastes. The thick film pastes are screen printed onto substrates forming conductive circuit patterns. These circuits are then dried and fired to volatilize the liquid organic vehicle and sinter the silver particles.
Printed circuit technology is requiring denser and more precise electronic circuits. To meet these requirements, the conductive lines have become more narrow in width with smaller distances between lines. The silver powders necessary to form dense, closely packed, narrow lines must be as close as possible to monosized, smooth spheres.
Many methods currently used to manufacture metal powders can be applied to the production of silver powders. For example, chemical reduction methods, physical processes such as atomization or milling, thermal decomposition and electrochemical processes can be used.
Silver powders used in electronic applications are generally manufactured using chemical precipitation processes. Silver powder is produced by chemical reduction in which an aqueous solution of a soluble salt of silver is reacted with an appropriate reducing agent under conditions such that silver powder can be precipitated. The most common silver salt used is silver nitrate. Inorganic reducing agents including hydrazine, sulfite salts and formate salts can produce powders which are very coarse in size, are irregularly shaped and have a large particle size distribution due to aggregation.
Organic reducing agents such as alcohols, sugars or aldehydes are used with alkali hydroxides to reduce silver nitrate. The reduction reaction is very fast and hard to control and produces a powder contaminated with residual alkali ions. Although small in size (&lt;1 micron), these powders tend to have an irregular shape with a wide distribution of particle sizes that do not pack well.
The atomization method for making silver particles is an aerosol decomposition process which involves the conversion of a precursor solution to a powder. The process involves the generation of droplets, transport of the droplets with a gas into a heated reactor, the removal of the solvent by evaporation, the decomposition of the salt to form a porous solid particle, and then the densification of the particle to give fully dense spherical pure particles. Conditions are such that there is no interaction of droplet-to-droplet or particle-to-particle and there is no chemical interaction of the droplets or particles with the carrier gas.
The major problem that has heretofore limited successful application of this technique for powder generation is lack of control over particle morphology. In particular, it is the requirement that the material has to be treated above its melting point to form fully dense particles. Material treated below the melting point has tended to give impure, hollow-type particles which are not densified.